Abstract

DNA restriction maps of the major histocompatibility complex
and hybridization with low copy probes have previously
revealed strong homology between the Q6-Q7 and the Q8-Q9
class I gene pairs in the Qa2 region of the C57BL/10 mouse.
After DNA sequence analysis of the Q7, Q8 and Q9 genes,
we have compared the Q7 gene with its apparent allele, 27.1,
from the BALB/c mouse; the 99% homology between Q7 and
27.1 indicates that this is a non-polymorphic gene. Comparison
of Q7 with Q9, its homologue in the Q8-Q9 gene pair,
revealed > 99% homology, thus supporting our proposal that
the Qa2 region has evolved by the duplication of gene pairs.
Q7 was also found to be homologous (93%) to Q8, the second
member of the Q8-Q9 pair. However, the first exon (encoding
the leader sequence) as well as the first intron of Q7
and Q8, which are presumably not subject to strong selective
pressure, are essentially identical in nucleotide sequence
(having only one mismatch), which suggests that >200 bp
of DNA may have been exchanged by gene conversion. Furthermore,
transcripts of both Q7 and Q8 would have termination
codons derived from the exon that normally encodes the
transmembrane domain, thus these genes could encode either
membrane-bound class I proteins that lack a cytoplasmic protein
domain or class I proteins that are secreted.